In the phone system related to the present invention, the calling party phone-number notification service is provided in which the phone number of the phone terminal A of the calling party is notified to the phone terminal B of the called party. However, when it is desired not to show the actual phone number of the calling party phone terminal A on the called party, one possible solution is to notify another phone number to the called party's phone terminal B as the phone number of the calling party.
In the description below, another phone number used in place of the actual phone number of the calling party phone terminal A is called a “proxy phone number.”
One possible way to change the phone number of the calling party phone terminal A to a proxy phone number is that a call from the phone terminal A is terminated at the call relay server and, at the same time, the call relay server makes a call to the called party's phone terminal B using a proxy phone number.
Patent Document 1 describes a phone system that allows an external phone terminal to make a call to a desired extension phone terminal. The document shows an example of this system in which the call relay server (call relay device) makes a call to both the phone terminal of the call source and the extension phone terminal of the call destination (see FIG. 22 in Patent Document 1). In this case, it can be easily estimated that the calling party number the call relay server notifies the extension phone terminal is one of the extension phone numbers possessed by the call relay server.
FIG. 2 in Non-Patent Document 1 also shows a phone system of the third party call control type in which the controller, which serves as the call relay server, makes a call to both the calling party and the called party.
In any of the examples described above, when a call is connected from the call relay server to the called party phone terminal B, a proxy phone number corresponding to the calling party phone terminal A is used and, when a call corresponding to the proxy phone number is received as a callback call from the called party phone terminal B, the call relay device forwards the received call to the corresponding phone terminal A. In this way, the phone number of the phone terminal A can be hidden by the proxy phone number.
On the other hand, Patent Document 2 describes an invention that allows one user to have three phones each of which has the common proxy phone number (described as “transformed call originator number” in Patent Document 2).
According to this invention, when a call is made from any of the phones, the same call originator number (transformed call originator number) is displayed on a call destination phone and, when a call is returned from a call destination phone using the call originator number, a phone of the call destination is identified and the call is received by the extension phone that made the call.    [Patent Document 1] JP Patent No. 3904091    [Patent Document 2] Japanese Patent Publication Kokai No. JP-P2003-219445A    [Non-Patent Document 1] RFC3275, “Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)”